As stated in Section 18.104.22.168, the primary objective of (BA) modelling is to quantify the changes in regional groundwater due to , which is based on the difference in results between the (baseline) and (CRDP) simulations. A new regional-scale numerical groundwater model was deemed to be required for this purpose in the . The main objectives of the regional groundwater model are:
- to assess the at the and the corresponding year at which maximum change occurs by comparing the for the and futures
- to provide the change in – groundwater flux to the river model (reported in companion product 2.6.1 ( )).
A probabilistic approach of modelling is used which requires the groundwater model to be run thousands of times with different parameter combinations. This can have high computational overheads if the model domain is large and finely resolved. For the Namoi subregion, the modelling domain must encompass all the coal mines and coal seam gas (CSG) developments that are distributed across the subregion (see companion product 2.3 for more details on the coal resource development pathway ()). Given this large domain and the requirement to do thousands of simulations, the groundwater model must be computationally efficient, represent just the key processes for a regional-scale assessment and have a spatial resolution appropriate for representing local- to regional-scale effects of coal resource development.
The model needs to represent the maingroups that link mine and CSG development to groundwater responses on and off the mine sites (see companion product 2.3 for the Namoi subregion ( )):
- the ‘Subsurface depressurisation and dewatering’ causal pathway group, which involves subsurface and from the excavation of coal seams, mine water pumping and CSG developments
- the 'Subsurface physical flow paths’ causal pathway group, which involves changes in subsurface physical pathways due to hydraulic conductivity changes resulting from rock deformation due to mining
- the ‘Surface water drainage’ causal pathway group, which involves changes to surface water drainage through its interaction with groundwater.
Key outputs from the model are groundwater drawdowns and changes in surface water – groundwater exchanges, which are summarised as changes in key groundwater and surface water 22.214.171.124.2.1).at model nodes across the modelling domain (Section
Drawdowns due to additional coal resource development are reported asof the differences in drawdown between the CRDP and baseline simulations. The drawdowns are reported by the groundwater model at each model node in the model domain, but since most access water that is at or near the ground surface in the alluvium and outcropping areas of other geological units, these shallow model nodes that comprise the are the model nodes of greatest interest. In addition to the drawdown due to additional coal resource development at the model nodes, probabilistic maps of the and under the CRDP are presented, as well as maps of the difference between the two futures for the regional watertable.
The groundwater model is also used to estimate the change in surface water – groundwater flux to propagate to the surface water models (companion product 2.6.1 for the Namoi subregion (). The changes in from the CRDP and baseline simulations are used in the Namoi river model, where they are incorporated into the hydrological response variables generated as part of the surface water modelling ( ).
Since the main objective of the 126.96.36.199.2). The principle of superposition enables the modelling to focus on the change in hydrogeological stress and the hydraulic properties, rather than on reproducing historical conditions or predicting future state variables of the system, such as groundwater levels or fluxes.numerical modelling is to quantify the difference between two modelled futures, the emphasis on producing a well-calibrated model is lower than if the objective were to predict the state of groundwater under baseline and under CRDP (Section
Probabilistic estimates of CSG water production rates and mine are also provided by the groundwater model that are used in comparing the results to the water pumped reported by the proponents’ models (see Section 188.8.131.52). A more comprehensive discussion on the water balance components of the Namoi subregion is presented in companion product 2.5 ().
Product Finalisation date
- 184.108.40.206 Methods
- 220.127.116.11 Review of existing models
- 18.104.22.168 Model development
- 22.214.171.124 Boundary and initial conditions
- 126.96.36.199 Implementation of the coal resource development pathway
- 188.8.131.52 Parameterisation
- 184.108.40.206 Observations and predictions
- 220.127.116.11 Uncertainty analysis
- 18.104.22.168 Limitations
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product